a) Field of the Invention
The invention is directed to a crane comprising a traveling gear for moving along rails which has a plurality of traveling gear groups, each having at least two wheels which are rotatably mounted at a subframe so as to be spaced apart from one another in longitudinal direction of the respective rail and which are connected to an end carriage of the crane by a central joint having at least degrees of freedom for rotating the traveling gear group relative to the end carriage around a vertical axis and for swiveling the traveling gear group relative to the end carriage around a horizontal axis extending right angles to the rails.
b) Description of the Related Art
Traveling gear of rail-bound cranes, particularly gantry cranes or bridge cranes, comprise at least two traveling gear groups, each of which has at least one wheel for moving the crane and each of which individually or all of which are outfitted with at least one driving motor for moving the crane. For example, a traveling gear group is arranged on all four legs of a gantry crane.
An exact orientation of the axles of the wheels relative to the rails is essential for low wear, above all in heavy cranes whose traveling gear groups have two or more wheels. The traveling gears themselves can be produced with very high accuracy as regards the axle positions and the position of the track guiding means. The traveling gear groups can be measured in the assembled state. In practice, measurement results are usually far below the positional tolerances predetermined by the relevant standards and practices.
In a conventional embodiment form of a crane whose traveling gear groups each have at least two or more wheels which are spaced apart from one another in longitudinal direction of the respective rail, the individual traveling gear groups are connected to an end carriage of the steel construction of the crane with only one degree of freedom, namely a swiveling around a horizontal axis extending transverse to the rails. In this case, the bore holes in the end carriage which receive the swiveling axle of the respective traveling gear group must be very precise, particularly with respect to the axial position transverse to the runway. These bore holes constitute interfaces between the mechanical construction of the traveling gear group and the steel construction of the crane frame, which has often proven problematic in practice. An inclined position of the axles can be brought about, for example, by sunlight which heats the end carriage in an uneven manner so that the axle positions change. Also, horizontal forces caused by skewed running, wind forces, and inertial forces are absorbed by only by a certain quantity of wheels. Accordingly, in traveling gears of this kind having multiple wheels, the horizontal forces that must also be absorbed by the rail construction are disadvantageously large. The occurring axial deviations and the unfavorable distribution of horizontal forces lead to bending in the traveling gear and, accordingly, to increased wear on the track guiding means, the wheel running surfaces, and the rails.
Constructions for reducing wear are already known, wherein an additional degree of freedom is provided for the articulation of the traveling gear group at the end carriage, namely, rotatablity around a vertical axis. Accordingly, the traveling gears are free from bending relative to the stiff, imprecise steel construction of the crane, for example, a gantry crane. External forces due to differences in temperature and deformations no longer affect the running geometry. Horizontal forces are transmitted to the rails by practically all of the track guiding means. The oblique running behavior is substantially improved or oblique running can be ruled out in cranes with synchronized running devices.
Different constructions are already known for forming the connection between the traveling gear group and the end carriage with an additional rotatability around a vertical axis.
A construction using a live ring with balls or rollers is unobjectionable in technical respects but has a very large structural width and is very disadvantageous in terms of cost. The use of a sliding bearing support with an additional counter-support for absorbing tilting forces, also already known, has the disadvantage that very high rotational resistances must be overcome. The bearing forces can be 150 t or more, for example. Because of the high tilting forces, particularly when the corners of the crane are not loaded, very expensive counter-supports are provided in addition.
Further, it is known for this purpose to install an axial ball bearing with additional counterbearings. In this case, it is necessary to pretension the main bearing with very high pretensioning forces over the counterbearing until one-sided lifting of the main bearing is no longer possible. If the main bearing is lifted only slightly, the entire vertical force is shifted to one or a few rolling bodies which would lead to the destruction of the bearing. Disadvantages in this construction include the risk of a change in pretensioning forces, e.g., due to settling over the course of crane operation, loading of the balls and of the runway, which practically always occurs at the same point due to the very slight movement of the bearing, and the expensive construction.
In another known construction, a central joint in the form of a spherical pressure bearing which absorbs the bearing force is provided for connecting the traveling gear group to the end carriage. The tilting forces resulting from the horizontal forces transverse to the rail are absorbed by tension rods which are arranged at both sides of the central joint between the end carriage and the traveling gear group. One of the disadvantages is this case is that high bearing forces result from the tilting moment because the tension rods can only absorb tensile forces.